I'm fairly new to D&D 5e, but I have played some Pathfinder and 3.5e, so I have some experience with D&D. We are playing our current game in 5e by the rules as written.
Our DM had our party encounter a Mezzoloth (MM, p. 313) with 3-4 other enemies two sessions ago. We are level 6. We fled because we didn't prepare well enough for the fight and the terrain was working again us. At some point afterwards, he threw a cloudkill spell at us and we all nearly died, so we fled again.
Last game, we found the new location of the same Mezzoloth and decided to kill it, since the terrain was to our advantage this time (they were in a small room). About 16 hours elapsed between our two encounters.
The mezloth at some point cast cloudkill again. How does the mezzoloth's cloudkill spell work? The statblock says the mezzoloth can cast it with a frequency of "1/day". Does this mean it resets at midnight, or does it have to wait 24 hours between each use?